The invention relates to FM demodulators, and more particularly to digital FM demodulators, and most particularly to such demodulators employing the quadrature technique.
The principle of the quadrature demodulator is well-known in the art. In summary, the technique involves mixing an FM radio frequency signal with a version of the same signal which has been delayed 90.degree. at the center frequency (f.sub.o) of the FM signal. The resulting signal contains a DC component whose time varying level is representative of the original modulating signal. The digital version of the quadrature FM demodulator was disclosed by David Ludington in Electronic Design, Apr. 12, 1976, p. 126, which article is incorporated herein by reference.
The digital versions of the quadrature FM demodulator are subject to audio noise due to the use of clocked shift registers to provide the required 90.degree. phase shift. The noise results from an interaction between the shift register clock rate and the period of the FM signal being demodulated to produce beat frequencies, as will be more fully described hereinafter. This noise is, of course, detrimental to the operation of the demodulator, and it is desirable that it be removed.
It is therefore an object of the present invention to provide a digital FM demodulator which reduces the amplitudes of beat frequencies caused by digital sampling.